Metal foam heat sink

ABSTRACT

Further, a system is provided for dissipating heat from a semiconductor module including a semiconductor die and the unitary heat sink. The heat sink comprising a unitary body having both a porous and non-porous portion is provided. The non-porous portion is attached to the semiconductor die and configured to transfer heat to the porous portion for dissipation into the environment. In addition, a method for manufacturing the heat sink is provided.

BACKGROUND

1. Field of the Invention

The present invention generally relates to thermal management of asemiconductor device. More specifically, the invention relates to ametal foam heat sink for thermal management of a semiconductor device.

2. Description of Related Art

In electronic applications semiconductor devices can generatesignificant heat performing normal operations. This heat adverselyaffects the performance and reliability of the devices, if notdissipated. If the heat is not dissipated, the device may overheat suchthat the junction temperature increases to a level causing the device tofail or function improperly. Devices and interconnects may also fail dueto the effects of thermal expansion caused by the overheating. Forexample, stress caused by a mismatch in thermal expansion betweenmaterials can cause solder joint cracking. Therefore, it is advantageousto maximize the capability of a device to remove heat and to minimizethe effects of thermal expansion.

Heat dissipation from power devices is commonly accomplished with ametal heat sink, either on the top or bottom of the device. The heatsink is typically a metal block or sheet and may include fins. Finsprovide additional surface area for the dissipation of heat. Finstructures with high surface area, however, are bulky and expensiveoften requiring complex machining operations for fabrication.

Recently, metal foam has been used in place of fins to aid in thedissipation of heat. The porosity of the metal foam creates an enormoussurface area thereby providing high heat dissipation. Metal foam may beattached to a metal block or directly to the semiconductor device.However, utilizing metal foam in conjunction with a block heat sinkrequires an additional mechanical connection. The mechanical connectionmay be accomplished using solder or a layer of thermally conductiveadhesive. This additional connection increases thermal resistance andhinders effective dissipation of the heat.

In view of the above, it is apparent that there exists a need for a heatsink that provides improved heat dissipation.

SUMMARY

In satisfying the above need, as well as overcoming the enumerateddrawbacks and other limitations of the related art, the presentinvention provides a heat sink comprising a unitary body having bothfirst and second portions, a porous and non-porous portion. Thenon-porous portion provides for the transfer and spreading of heat whilethe porous portion provides for heat dissipation. When implemented in asemiconductor module, including a semiconductor die and the heat sink,the non-porous portion of the heat sink is attached to the semiconductordie and configured to transfer heat to the porous portion, whichdissipates the heat into the environment.

In yet another aspect of the invention, a method for manufacturing theheat sink is provided. The method includes the steps of forming a bodyhaving a first and a second portion, melting the second portion, andcreating porosity in the second portion. The heat sink is made of ametal material, preferably a copper alloy. The alloy content of theportions may be varied such that the melting temperature of the secondportion is lower than the melting temperature of the first portion. Thevarying melting temperature allows porosity to be created in the secondportion. This can be achieved by forcing gas through the second portionor by inserting a material into the second portion that may be removed,by burning or chemical reaction, after the second portion hassolidified.

Further objects, features and advantages of this invention will becomereadily apparent to persons skilled in the art after a review of thefollowing description, with reference to the drawings and claims thatare appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway side view of a heat dissipating system in accordancewith the present invention and implemented in connection with asemiconductor module.

DETAILED DESCRIPTION

Referring now to FIG. 1, a system embodying the principles of thepresent invention is illustrated therein and designated at 10. Thesystem 10 generally includes a semiconductor die 12 and a heat sink 14.

A semiconductor die 12 generates heat while performing its normaloperations. The die 12 is soldered to the metal vias 24. The metal vias24 transfer the heat generated by the die 12 to the heat sink 14 at theopposite side of the printed circuit board 22. The heat sink 14 isattached to the thermal vias 24 and the printed circuit board 22 by athermally conductive adhesive 20. However, other means of attachmentincluding solder may be utilized.

The heat sink 14 includes a solid non-porous portion 16 and a porousportion 18. The non-porous portion 16 provides a thermal mass for heatspreading or sinking. The non-porous portion 16 also provides theability to absorb short term transients allowing quick transfer of theheat away from the die. The porous portion 18 of the heat sink 14provides an extremely large surface area for dissipation of the heatinto the surrounding environment. Although, the heat sink 14 isdescribed as a porous portion 18 and a non-porous portion 16 the heatsink is a single unitary structure thereby eliminating mechanicalinterfaces which may increase thermal resistance.

Natural convection may be used to dissipate heat from the porous portion18 of the heat sink 14. However, air or liquid may also be forcedthrough the porous section 18 of the heat sink 14. The flow of the gasor liquid cooling is illustrated by arrow 28. The heat sink 14 ispreferably made of copper although aluminum or other metals may be used.In addition, the pore sizes and the thickness of each portion may bemanipulated based on the package size and amount of heat to bedissipated.

In addition, a method for manufacturing the heat sink is provided. Theheat sink will have a unitary body including a first and second portion.The first portion can be a solid non-porous metal block, and may includea higher alloy content thereby causing the first portion to have ahigher melting temperature than the second portion. The second portionmay be melted at a temperature such that the first portion remainssolid. A porosity is then created in the melted or second portion of theheat sink. The porosity may be created by forcing gas through the meltedportion. Alternatively, a foreign material may be inserted into themelted portion. With the foreign material integrated, the second portionmay be allowed to solidify creating a porous surface area of the secondportion. After solidification of the second portion, the foreignmaterial may be removed by burning, chemical vaporization or othermethods. Additional manufacturing operations may then be performed tothe heat sink including milling, drilling, or similar operations.

As a person skilled in the art will readily appreciate, the abovedescription is meant as an illustration of implementation of theprinciples this invention. This description is not intended to limit thescope or application of this invention in that the invention issusceptible to modification, variation and change, without departingfrom spirit of this invention, as defined in the following claims.

1. A heat sink comprising a unitary body having first and secondportions, the first portion being generally non-porous and the secondportion being generally porous; whereby the first portion transfers andspreads heat within the heat sink and the second portion substantiallydissipates the heat from the heat sink.
 2. The heat sink according toclaim 1, wherein the first and second portions are made of a metalmaterial.
 3. The heat sink according to claim 1, wherein the first andsecond portions include a copper alloy.
 4. The heat sink according toclaim 1, wherein the first portion is generally solid.
 5. The heat sinkaccording to claim 1, wherein the second portion has a meltingtemperature that is lower than a melting temperature of the firstportion.
 6. A system for dissipating heat comprising: a semiconductordie; and a unitary heat sink attached to the semiconductor die, the heatsink including a non-porous portion and a porous portion.
 7. The systemaccording to claim 6, wherein the semiconductor die is soldered to thenon-porous portion of the unitary heat sink.
 8. The system according toclaim 6, wherein the heat sink is made of a copper alloy.
 9. The systemaccording to claim 6, wherein a melting temperature of the porousportion is lower than a melting temperature of the non-porous portion.10. The system according to claim 6, wherein a gas is forced through theporous portion of the heat sink.
 11. The system according to claim 6,wherein a liquid is forced through the porous portion of the heat sink.12. The system according to claim 11, wherein the fluid is a dielectricfluid.
 13. A method for manufacturing a heat sink comprising the stepsof: forming a unitary body having a first portion therein and a secondportion; melting the second portion of the body; and creating porosityin the second portion of the body.
 14. The method according to claim 13,wherein the first portion has a higher melting temperature than thesecond portion.
 15. The method according to claim 13, wherein theporosity is created by forcing a gas through the second portion.
 16. Themethod according to claim 13, wherein the step of creating porosityincludes the step of integrating a material into the second portion. 17.The method according to claim 16, further comprising the step ofsolidifying the second portion with the material integrated therein. 18.The method according to claim 17, further comprising the step ofremoving the material from the second portion.
 19. The method accordingto claim 18, wherein the material is removed by a chemical interaction.